1. Technical Field
The present invention generally relates to lubricating oil compositions containing at least a thio-functionalized phenylenediamine as an antioxidant.
2. Description of the Related Art
Thio-functionalized phenylenediamine compounds are known. See, e.g., U.S. Pat. No. 4,072,654 and International Publication Nos. WO 02/42262 and WO 04/031287. These compounds are indicated as being useful as stabilizers for elastomers to prevent oxidative, thermal, dynamic, light-induced and/or ozone induced degradation. They are also indicated as being suitable as stabilizers for elastomers to prevent contact discoloration of substrates coming into contact with elastomers.
In developing lubricating oils, there have been many attempts to provide additives that impart, for example, antioxidant, antiwear, and deposit control properties thereto. Zinc dialkyldithiophosphates (ZDDP) have been used as antifatigue, antiwear, antioxidant, extreme pressure and friction modifying additives for lubricating oils for many years. However, they are subject to several drawbacks owing to their zinc and phosphorus contents. The presence of zinc contributes to the emission of particulates in the exhaust. In addition, during operation of an internal combustion engine, lubricating oil enters the combustion chambers by means such as clinging to cylinder walls as the piston makes its down stroke.
When phosphorus-containing lubricating oil compositions enter the combustion reaction, phosphorus enters the exhaust stream where it acts as a catalyst poison thus shortening the useful life of the catalytic converter. Moreover, zinc dialkyldithiophosphates give rise to ash, which contributes to particulate matter in automotive exhaust emissions, and regulatory agencies are seeking to reduce emissions of zinc into the environment. Thus, it is not only important to limit the particulate matter and pollution formed during engine use for toxicological and environmental reasons, but it is also important to maintain the antioxidant properties of the lubricating oil.
In view of the aforementioned shortcomings of the known zinc and phosphorus-containing additives, efforts have been made to provide lubricating oil additives that contain neither zinc nor phosphorus or, at least, contain them in substantially reduced amounts.
It would therefore be desirable to provide lubricating oil compositions having improved properties while also having a reduced content of zinc and phosphorous.